Various types of valve apparatus are used in various wellbore types (e.g., subsea, platform, land-based) to control fluid flow through tubing or conduits disposed therein. One such valve is referred to as a subsurface safety valve, or simply as a safety valve, and it provides a “fail-safe” mechanism for closing the wellbore to prevent the uncontrolled release of hydrocarbons or other downhole fluids. Such safety valves are typically actuated in emergency situations, such as blowouts, to provide a pressure barrier (oftentimes in cooperation with blowout preventers) and safeguard local personnel, equipment, and the environment.
U.S. Pat. No. 4,161,219 discloses a safety valve configuration that employs a flapper valve that is spring-biased towards a position closing a fluid passageway in the safety valve body, and a flow tube that is movable between a first position yielding the biasing spring of the flapper valve to open the flapper valve and a second position permitting the biasing spring of the flapper valve to close the flapper valve. The flow tube is also spring biased towards the second position that releases the flapper valve, but the flow tube is normally urged towards the first position in which the flapper valve is opened by the application of hydraulic fluid pressure from the surface. In the event of an emergency, such as a blowout, the hydraulic fluid pressure is reduced to permit the spring bias of the flow tube to urge the flow tube towards its second position, thereby releasing the flapper valve so that its biasing spring urges the flapper valve towards the position closing the fluid passageway.
It is commonly believed today that most of the remaining oil and gas reserves of considerable substance are located in so-called “deep water” or “ultra-deep water” subsurface formations. Such formations may lie underneath 7000 feet or more of water and up to 30,000 feet or more beneath the seafloor. Some industry experts predict that by the year 2015, 25% or more of offshore oil production will be sourced from deepwater wellbores. As deepwater wells are drilled to greater depths, they begin to encounter extreme high pressure, high temperature conditions (i.e., having an initial reservoir pressure greater than approximately 10 kpsi (69 Mpa) or reservoir temperature greater than approximately 300° F. (149° C.)) that constitute one of the greatest technical challenges facing the oil and gas industry today. As. a result, materials that have been used for many years now face unique and critical environmental conditions for which they may not be suitable.
A clear example of such material challenges is found in hydraulic fluids, which are used in a number of downhole applications including safety valve actuation as described above. Hydraulic fluids will suffer a breakdown or stagnation when exposed to high temperatures over time (safety valves can sit dormant downhole for decades) that severely compromises the hydraulic properties of such fluids, rendering them incapable of functioning for their intended hydraulic purposes. Additionally, hydraulically-actuated safety valves are subject to seal failure over time that reduces their performance and reliability.
Therefore, a need exists for a means of reliably actuating valves such as safety valves in downhole environments, particularly in the high pressure, high temperature environments of deepwater wellbores.